noun · a thin-walled, air-filled cavity in the body of an animal or plant, especially in birds

鸟类的气囊可以帮助它们在飞行时减轻体重并辅助呼吸。

noun · a flexible container filled with gas in an aerostat, or a safety airbag in a vehicle

安全气囊在车辆发生碰撞时会迅速充气弹出。